A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting. Accordingly, there is an ongoing need to provide lighting which is more energy-efficient. It is well-known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10) but are still less efficient as compared to solid state light emitting devices, e.g., light emitting diodes.
In addition, as compared to the normal lifetimes of light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, the lifetime of light emitting diodes, for example, can generally be measured in decades. Fluorescent bulbs have longer lifetimes (e.g., 10,000-20,000 hours) than incandescent lights, but provide less favorable color reproduction. Color reproduction is typically measured using the Color Rendering Index (CRI Ra) which is a relative measure of the shift in surface color of an object when lit by a particular lamp. Daylight has the highest CRI (Ra of 100), with incandescent bulbs being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower).
Another issue faced by conventional light fixtures is the need to periodically replace the lighting devices (e.g., light bulbs, etc.). Such issues are particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, traffic tunnels) and/or where change-out costs are extremely high. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Light-producing device lifetime is typically much shorter, thus creating the need for periodic change-outs.
Accordingly, for these and other reasons, efforts have been ongoing to develop ways by which solid state light emitters, e.g., light emitting diodes, can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide light emitting diodes (or other solid state light emitters) which are improved, e.g., with respect to energy efficiency, color rendering index (CRI Ra), contrast, efficacy (lm/W), and/or duration of service.
Light emitting diodes are semiconductor devices that convert electrical current into light. A wide variety of light emitting diodes are used in increasingly diverse fields for an ever-expanding range of purposes.
More specifically, light emitting diodes are semiconducting devices that emit light (ultraviolet, visible, or infrared) when a potential difference is applied across a p-n junction structure. There are a number of well-known ways to make light emitting diodes and many associated structures, and the present invention can employ any such devices. By way of example, Chapters 12-14 of Sze, Physics of Semiconductor Devices, (2d Ed. 1981) and Chapter 7 of Sze, Modern Semiconductor Device Physics (1998) describe a variety of photonic devices, including light emitting diodes.
The expression “light emitting diode” is used herein to refer to the basic semiconductor diode structure (i.e., the chip). The commonly recognized and commercially available “LED” that is sold (for example) in electronics stores typically represents a “packaged” device made up of a number of parts. These packaged devices typically include a semiconductor-based light emitting diode such as (but not limited to) those described in U.S. Pat. Nos. 4,918,487; 5,631,190; and 5,912,477; various wire connections, and a package that encapsulates the light emitting diode.
As is well-known, a light emitting diode produces light by exciting electrons across the band gap between a conduction band and a valence band of a semiconductor active (light-emitting) layer. The electron transition generates light at a wavelength that depends on the band gap. Thus, the color of the light (wavelength) emitted by a light emitting diode depends on the semiconductor materials of the active layers of the light emitting diode.
Although the development of solid state light emitters has in many ways revolutionized the lighting industry, some of the characteristics of solid state light emitters have presented challenges, some of which have not yet been fully met. For example, the emission spectrum of any particular light emitting diode is typically concentrated around a single wavelength (as dictated by the light emitting diode's composition and structure), which is desirable for some applications, but not desirable for others, (e.g., for providing lighting, such an emission spectrum provides a very low CRI).
Because light that is perceived as white is necessarily a blend of light of two or more colors (or wavelengths), no single light emitting diode junction has been developed that can produce white light. “White” light emitting diodes have been produced which have a light emitting diode pixel formed of respective red, green and blue light emitting diodes. Other “white” light emitting diodes have been produced which include (1) a light emitting diode which generates blue light and (2) a luminescent material (e.g., a phosphor) that emits yellow light in response to excitation by light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light.
In addition, the blending of primary colors to produce combinations of non-primary colors is generally well understood in this and other arts. In general, the 1931 CIE Chromaticity Diagram (an international standard for primary colors established in 1931), and the 1976 CIE Chromaticity Diagram (similar to the 1931 Diagram but modified such that similar distances on the Diagram represent similar perceived differences in color) provide useful reference for defining colors as weighted sums of primary colors.
Solid state light emitters can thus be used individually or in any combinations, optionally together with one or more luminescent material (e.g., phosphors or scintillators) and/or filters, to generate light of any desired perceived color (including white). Accordingly, the areas in which efforts are being made to replace existing light sources with solid state light emitters, e.g., to improve energy efficiency, color rendering index (CRI), efficacy (lm/W), and/or duration of service, are not limited to any particular color or color blends of light.
A wide variety of luminescent materials (also known as lumiphors or luminophoric media, e.g., as disclosed in U.S. Pat. No. 6,600,175, the entirety of which is hereby incorporated by reference) are well-known and available to persons of skill in the art. For example, a phosphor is a luminescent material that emits a responsive radiation (e.g., visible light) when excited by a source of exciting radiation. In many instances, the responsive radiation has a wavelength which is different from the wavelength of the exciting radiation. Other examples of luminescent materials include scintillators, day glow tapes and inks which glow in the visible spectrum upon illumination with ultraviolet light.
Luminescent materials can be categorized as being down-converting, i.e., a material which converts photons to a lower energy level (longer wavelength) or up-converting, i.e., a material which converts light including photons of an energy level to light including photons of a higher energy level (shorter wavelength).
Inclusion of luminescent materials in LED devices has been accomplished by adding the luminescent materials to a clear plastic encapsulant material (e.g., epoxy-based or silicone-based material) as discussed above, for example by a blending or coating process.
For example, U.S. Pat. No. 6,963,166 (Yano '166) discloses that a conventional light emitting diode lamp includes a light emitting diode chip, a bullet-shaped transparent housing to cover the light emitting diode chip, leads to supply current to the light emitting diode chip, and a cup reflector for reflecting the emission of the light emitting diode chip in a uniform direction, in which the light emitting diode chip is encapsulated with a first resin portion, which is further encapsulated with a second resin portion. According to Yano '166, the first resin portion is obtained by filling the cup reflector with a resin material and curing it after the light emitting diode chip has been mounted onto the bottom of the cup reflector and then has had its cathode and anode electrodes electrically connected to the leads by way of wires. According to Yano '166, a phosphor is dispersed in the first resin portion so as to be excited with the light A that has been emitted from the light emitting diode chip, the excited phosphor produces fluorescence (“light B”) that has a longer wavelength than the light A, a portion of the light A is transmitted through the first resin portion including the phosphor, and as a result, light C, as a mixture of the light A and light B, is used as illumination.
As noted above, “white LED lights” (i.e., lights which are perceived as being white or near-white) have been investigated as potential replacements for white incandescent lamps. A representative example of a white LED lamp includes a package of a blue light emitting diode chip, made of gallium nitride (GaN), coated with a phosphor such as YAG. In such an LED lamp, the blue light emitting diode chip produces an emission with a wavelength of about 450 nm, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nm on receiving that emission. For instance, in some designs, white light emitting diodes are fabricated by forming a ceramic phosphor layer on the output surface of a blue light-emitting semiconductor light emitting diode. Part of the blue ray emitted from the light emitting diode chip passes through the phosphor, while part of the blue ray emitted from the light emitting diode chip is absorbed by the phosphor, which becomes excited and emits a yellow ray. The part of the blue light emitted by the light emitting diode which is transmitted through the phosphor is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light.
Designs have been provided in which existing LED component packages and other electronics are assembled into a fixture. In such designs, a packaged LED is mounted to a circuit board, the circuit board is mounted to a heat sink, and the heat sink is mounted to the fixture housing along with required drive electronics. In many cases, additional optics (secondary to the package parts) are also necessary.
There is an ongoing need for ways to use solid state light emitters in a wider variety of applications, with greater energy efficiency, with improved color rendering index (CRI), with improved efficacy (lm/W), and/or with longer duration of service.